In a RFID using a contactless power supply, a contactless power reception circuit is provided in a wireless tag side. The contactless power reception circuit receives power from an adjacent electromagnetic field of a reader-writer for example, by coupling, and drives an internal circuit thereof. In such a RFID system, the wireless tags may transmit signals by a load modulation communication to the reader-writer for the purpose of an authentication. See, Japanese Unexamined Patent Application Publication No. 2007-288718.
FIG. 1 is a circuit diagram showing an example of rough configuration for a conventional contactless power reception circuit using the load modulation communication.
A receiver part 102 includes a coil antenna and is coupled with the adjacent electromagnetic field of the reader-writer. A rectifier part 103 includes a diode and rectifies a generated voltage signal at the receiver part 102. A voltage regulator 104 regulates a rectified voltage signal at the rectifier 103, and transforms to a specified voltage. A power supply part 105 utilizes an output voltage from the voltage regulator 104 as a driving voltage for an internal circuit. DATA reception part 107 demodulates a reception signal from the reader-writer. A clock generator 108 generates a clock signal based on the reception signal from the reader-writer. A signal processor 109 generates a transmission signal to the reader-writer. A load modulator part 106 switches a load value based on the transmission signal generated at the signal processor 109. By switching the load value, status of a power supply from the adjacent electromagnetic field changes, and a voltage level of the coil antenna at reader-writer side changes. Accordingly, the reader-writer can detect the transmission signal from the wireless tags based on changes of the voltage level. At the wireless tags, only a power supply enough for driving the signal processor is required, thus a power transmitted from the voltage regulator 104 to the power supply part 105 is in the order of mW.